In modern fiberglass manufacturing, large glass melting furnaces are employed to melt the glass making ingredients. The furnaces are typically elongated, rectangular shaped brick structures lined internally with specialized refractories. Glass making ingredients are fed into one end of the furnace and molten glass is removed from the other end and passed to a forehearth. The forehearth is typically of considerable length and has attached to its underside a plurality of fiber forming bushings or stream feeders through which the molten glass in the forehearth flows to form the glass fibers. Gas burners are provided along the length of the forehearth to heat the molten glass as it passes from the furnace through the forehearth to the bushings and to maintain it at satisfactory temperatures to provide the optimum viscosity for the glass as it passes through the bushings for fiber formation. In the textbook, "The Manufacturing Technology of Continuous Glass Fibers", by K. L. Lowenstein, Elsiver Scientific Publishing Co., New York, N.Y., 1973, typical furnaces used for producing glass fibers are described in detail on pages 40-56. Similarly, the typical forehearths used in direct melt operations today, and their energy supply systems, are described on pages 60-70.
The bushings used to form the fibers are electrically resistance heated platinum or platinum alloy structures which need transformers attached to them by suitable connections to terminals provided on the bushings to supply power to them. In addition, water cooling is required to seal the glass around the bushing assembly attached to the forehearth and thereby eliminate glass leaks. The terminal clamps for transformer connection require cooling and the nozzle shields or fin coolers which are typically placed between the rows of forming tips on the fiber forming bushing base plates also require cooling. The fin cooler or nozzle shields are described by the aforementioned Lowenstein text on pages 106-110 and a typical water cooled bushing terminal clamp is depicted in FIG. v/17 on page 116 of the same reference.
Experience has shown that it has been impossible to obtain uniform heat at every aperture in a fiber forming bushing because of their shape and the environment variations inherent in the construction of the bushings and associated equipment as well as the competing cooling systems used to try and control the thermal environment as the fibers are formed and collected. Non-uniform temperatures at the fiber forming tips on a fiber glass bushing result in fiber diameter variations that can be as much as 2 to 1 between tips. This represents a 4 to 1 variation in mass.
Little change has occurred in the fiber glass industry over this type of glass forming operation except that the furnaces have become bigger, forehearths longer and the number of bushings per furnace has been increased as well as individual bushing size in terms of number of forming tips per bushing. This expansion in size has meant development and use of more expensive temperature controls for the resistance heated bushings, bulkier transformers for current supply, more sophisticated and controlled cooling systems for the transformer clamps on the bushings, the bushing fin coolers used adjacent the bushing tips and the reactors used to regulate the energy supplied to the resistance heated bushings.
Still with all of this expenditure of capital the variations at the fiber forming tips while improved over the earliest days of the industry still leave a lot to be desired. Thus the bushings still experience variations in fiber diameter from tip to tip that can be improved upon considerably. Further, the electrical systems and cooling requirements necessary to achieve the current and less than ideal variation in fiber diameters during formation involve large capital expenditures and considerable amounts of energy all of which can be saved with a more efficient melting and forming process.
In U.S. Pat. No. 3,285,720 a new approach to feeding glass is described using a siphon bushing from a hot glass melt. A much more uniform fiber pack in a strand was observed with this process because of the more uniform temperatures in the tip plate. When this was attempted with a series of siphon bushings along a flow channel, the output from one bushing to another was found to be different, however. Since the flow channel had no internal method for controlling temperature along the entire channel, output differences were determined to be due to variations in glass viscosity along the glass channel.
In another U.S. Pat. No. 4,337,073 a process is described in which a circular forehearth is used to feed glass to bushings positioned thereon as opposed to the typical elongated forehearth and bushing arrangements used in the industry today. This process involved feeding molten glass to a rotating forehearth section which in turn streamed uniform hot glass into a chamber in which unheated (i.e. no resistance heating applied) bushings were attached. The rotary forehearth section was heated by central gas burner and internal radial gas passages located in the rotor. While some uniformity of fibers was demonstrated by this process, the forehearth arrangement was intricate and required a gas distribution system in the rotor to provide uniform temperatures throughout from a central source.
Thus, despite all of the efforts made to date, a need still exists to provide a more simple approach to glass fiber forming which will deliver quality uniform fibers while minimizing investment and reducing energy requirements for fiberization. The applicant's novel process provides a method of accomplishing these goals.